Most refrigerant air conditioners rely on the ‘refrigeration cycle’ generally comprising four standard processes:
1) The refrigerant starts as a vapor at low pressure inside an electrical motor driven compressor. Pressure is increased which increases the temperature of the refrigerant vapor as it compresses and flows toward a condenser.
2) Inside the condenser, heat is released from the high pressure refrigerant to the outside air due to a temperature gradient, causing the refrigerant to condense and become a high pressure, high temperature liquid.
3) The refrigerant next flows towards a pressure regulation valve, which causes an adiabatic expansion of the refrigerant, causing a phase change to vapor, causing the temperature of the refrigerant to drop below the temperature of the refrigerated space resulting in a cold, low pressure vapor.
4) The cold refrigerant vapor flows to the evaporator where it absorbs heat from the indoor air to the refrigerant. The warmed vapor flows back to the compressor where the cycle is repeated.
Typically, the condenser is powered by electricity, and most commercial air conditioners have an energy-efficiency rating that lists how much heat (measured in BTU per hour) is removed for each watt of power the air conditioner draws. These efficiencies improve with more efficient compressors, larger and more effective heat exchanger surfaces, improved refrigerant flow and other features.
The present invention shows advantages over other refrigeration systems in that system is mechanical, using a piston to perform the duties commonly associated with a compressor and an evaporator while drawing energy from low grade waste heat energy without significant work done by electricity for cooling. Further, the preferred embodiment runs directly from solar energy which is concentrated by a U-tube type concentrator to power the refrigeration system.
Overview of A/C System
The present refrigeration system takes in refrigerant starting in vapor form, and compresses it during a heat pump cycle. The pressurized refrigerant flows through a refrigeration inlet valve into a condensation unit containing a heat exchanger. The heat exchanger removes heat from the refrigerant causing it to condense. The condensed refrigerant then collects into a condenser tank. The condenser tank is connected with an evaporator tank through a pressure regulator. The evaporator tank is also in connection with a heat exchanger forming a loop for receiving heat from the enclosure to be cooled, such as a building. Additionally a pre-heater may be added between the condenser tank and the evaporator tank aid in heat transfer.
The loop for receiving heat from the space to be cooled is formed in this instance by a pipe having heat exchanger fluid, forming a heat exchanger loop between the evaporator tank heat exchanger and the enclosure heat exchanger, located inside the enclosure to be cooled. The cold reservoir of condensed refrigerant inside the evaporator tank cools the evaporator tank heat exchanger. The warmer air of the enclosure transfers via the enclosure heat exchanger into the system.
The compression and expansion stages can be accomplished using one element within the preferred embodiment of a U-tube concentrator; the compression and expansion strokes of the liquid piston. Those skilled in the art may devise other means for providing compression and expansion at one location, by using a pump, piston or similar means not connected with a liquid piston which does not depart from the present invention.
U-Tube as Solar Concentrator
Three major technologies are currently being used for concentrating solar heat generation to produce useful work; the parabolic trough, the power tower, and the sterling dish. The costs of generating electricity from these power sources are high. All three require a high working temperature, which creates problems with maintenance and high seal failure rates.
With these technologies, the solar radiation is concentrated real time under direct sunlight resulting in high working temperature at the point of collection. This higher temperature generally leads to higher thermal losses. In addition, the high temperature requirements of these systems for minimal thermal loss typically forces the use of more expensive and complicated collectors and thermal storage units. This constraint leads to higher costs for these solutions.
With the advent of low temperature solar concentrators such as those disclosed in U.S. patent application Ser. No. 11/387,405, and U.S. patent application Ser. No. 11/860,506 both included herein by reference, it is desirable to minimize condensation from saturated vapors associated with thermodynamic cycles in the heat engine cycle which run at or near the phase change point. Such improvements increase the efficiency and allow use of a lower grade of thermal energy.
The preferred embodiment utilizes a dual loop U, or other suitably formed heat actuated liquid piston heat pump, where one leg comprises a heat engine and the other leg comprises a heat pump. Those skilled in the art will appreciate that it can broadly be applied to any method or apparatus which runs a thermodynamic cycle at or near the condensation point of a vapor.
These floating pistons are usually constructed from a solid material, for example, aluminum, non corrosive steel, or other suitable material. They should be designed to withstand the conditions of temperature and pressure found in the system.
The heat engine section operates using a thermodynamic cycle from a natural or waste heat source, such as, but not limited to, solar energy. Fluid, typically water, in the liquid or steam form, is transferred between the solar collectors and the heat engine as part of the heat engine loop.
The heat pump loop is connected to the outlet and inlet of the refrigeration system and the heat pump expansion chamber is substantially filled with a refrigerant typically in a substantially vapor form.
A further advantage of the present invention is that the refrigeration increases with higher ambient heat, when it is most needed. This increase in output comes from several factors, but the most significant, is the temperature-pressure characteristics of the steam used in the U-tube concentrator. When used with flat panel solar collectors, the available steam input temperature increases with ambient temperature because the collector losses to ambient decrease as the ambient temperature rises.
As a reference, at a steam input temperature of 170° F.; 6 psig is available for the down stroke of the heat engine piston. At a steam input temperature of 200° F.; 11.5 psig is available. Since the power available from the heat engine is proportional to the steam pressure, this provides a substantial increase in power.
Further, the corresponding exhaust pressure does not rise proportionately. As useful work is a function of the difference between steam input steam temperature and ambient output temperature, a rise in output temperature robs system power. However, increase in rejection temperature causes a much smaller increase in exhaust pressure and a correspondingly smaller decrease in power compared with the gains at the input. For example, at a 100° F. exhaust temperature, the exhaust pressure is 0.9 psi. For a 130° F. exhaust temperature, the exhaust pressure only increases to 2.2 psi.
It should be noted that the present system can operate at much lower temperatures than previous systems and can be scaled as temperature rises. The same conditions causing a need for increased cooling, intense sunlight and heat also improves output capacity of the system. Additionally as conditions moderate, output is reduced with demand, but the system can even operate with heat input from thermal storage collected during peak hours. This feature offers a tremendous advantage over other systems that can only work under direct solar radiation.
The Heat Engine Cycle of the Preferred Embodiment (Water)
The isentropic compression process of the typical Carnot cycle starts with a working fluid such as water in the steam phase and ends with liquid phase. Whereas the present cycle starts with wet steam and ends with saturated vapor. The disclosed process is relatively unintuitive because condensation from a vapor to a liquid is commonly associated with a compression process.
In the present cycle, the compression process is constrained to form saturated vapor to maintain constant entropy as required by the process.
In the present embodiment, only approximately 12.5% of the wet steam mixture is liquid at the beginning of the compression process. At the beginning of the process, the specific entropy of the liquid is approximately 0.53 kJ/kg-° K and the specific entropy of the vapor is approximately 8.32 kJ/kg-° K. At the end of the compression process, the specific entropy of the liquid is approximately 1.31 kJ/kg-° K and the specific entropy of the vapor is approximately 7.36 kJ/kg-° K.
Quantitatively, an algebraic calculation equating total entropy at the beginning and end of the compression process with a single unknown of the amount of mass that changes between phases provides the result of vapor at the end of the cycle. Qualitatively, it can be seen that the relatively low percentage of liquid in the system at the beginning of the process drives the process to produce vapor. Because the majority of the system initially consists of high entropy vapor, converting all the vapor to liquid at approximately 16% of the specific entropy cannot be a constant entropy process. However, if the process produces vapor at approximately 88% of the initial vapor specific entropy, constant entropy can be maintained, with the approximately 13.9 times increase in the liquid to vapor entropy balancing the approximately 12% drop in the specific entropy of the initial vapor mass.
In a typical Carnot cycle that has a high initial percentage of liquid, the process is suboptimal. In this case, using the same starting and ending entropy values, the specific entropy of the majority of the mass, which is liquid, increases by a factor of approximately 2.5, if the final result is liquid. The mass of vapor that condenses drops in entropy by a factor of approximately 6.4 to balance out the increase in entropy of the liquid. The small drop in entropy of the initial vapor reduces the useful work which can be done by the system.
Therefore, it can be seen by one skilled in the art that there remains an incentive to maintain as much working fluid in the vapor phase as possible at the end of the process. By reducing the number of surfaces inside the chamber, including the piston head, where condensation can occur, this new cycle can be enabled with greater efficiencies as shown above.
The Heat Pump Cycle of the Preferred Embodiment
A refrigeration system can be attached to the heat pump side of the concentrator, which receives work done by the heat engine cycle. Those skilled in the art will recognize that other methods and apparatus can be used to generate similar types of work to operate an air conditioner system, while maintaining the spirit of the invention. The heat engine described herein is but one source of potential work to power a piston based refrigeration system.
The heat pump side of a U-tube concentrator contains the heat pump and the heat pump chamber, representing the heat pump cycle of the system. Further the U-tube concentrator operates with large volumes and low frequencies which is well suited to the compression and evaporation processes.
The heat pump loop is connected to the outlet and inlet of the refrigeration system and the heat pump chamber is filled with a refrigerant, such as HCFC-123, also known as “refrigerant-123” or “R123.” As can be appreciated, those skilled in the art may be able to use other refrigerants or working fluids without departing from the spirit of this invention.
The heat pump piston serves to separate the liquid connecting rod, typically water, from the refrigerant inside the heat pump chamber. The heat pump piston should be designed such that a seal is formed between the piston and the piston wall. An alternative embodiment of the U-tube concentrator allows the concentrator to operate a turbine or a refrigeration system. An additional inlet and outlet valve can be installed on the heat pump expansion chamber controlling the flow of the working fluid into a turbine attachment. The turbine could be designed to use the same energy source as the refrigeration system as disclosed. Energy allocated between the turbine and air conditioner may be controllable.
An additional advantage of using R-123 in the turbine instead of steam, results from turbine design parameters. For efficient design and operation of a vapor turbine, the optimal blade speed is proportional to the enthalpy change of the fluid as it passes through the stage. As a result, efficient turbine design requires tradeoffs between combinations of high blade speeds, large mass flow rates (high power), and small changes in enthalpy. This combination often results in large (1 to 500 MW) turbines or very high speeds (120,000 rpm) for small (30 to 100 kW) turbines. The enthalpy change of R-123 at typical concentrator output and input pressure is an order of magnitude less than the enthalpy change of steam for the same pressures. This allows for the selection of smaller power levels and lower speeds while maintaining the same turbine efficiency, making the system more suitable for distributed generation.
Another advantage of using R-123 in the turbine is that the fluid working temperature for the typical pressures can be 250° F. lower with R-123 than with steam. This provides substantial advantages in both the concentrator and the turbine in the areas of thermal expansion and material selection, particularly in the areas of seals and bearings.
A control system, typically electronically based, may be used to regulate the work distribution between the heat engine cycle and the heat pump cycle by receiving input from a variety of sensors along the concentrator and refrigeration system and controlling valves, pumps and the like at points along the system. A similar or separate control system may be used to allocate energy between the alternative turbine attachment and the refrigeration system.
It is an advantage of the invention that it cools an enclosure without being electrically powered, therefore not taxing existing electrical grid systems.
It is another advantage of the invention that is can cool an enclosure without creating a carbon foot print with regard to greenhouse gases.
It is another advantage of the invention that it combines the expansion and compression stage of the refrigeration cycle with one device.
It is another advantage of the invention that it scales, providing more cooling as temperatures rise.
It is another advantage of the invention that it is able to provide power using previously stored thermal energy.
It is another advantage of the invention it operates efficiently under high ambient temperatures (above 100° F.).
It is another advantage of the invention that waste heat rejected into the environment by ambient air cooling.
It is another advantage of the invention the waste heat rejected into the environment does not require evaporative cooling.
It is another advantage of the invention that it is powered by a U-tube concentrator.
It is an advantage of the invention that the invention can share power between a turbine and a refrigeration system.